QDoc: Clarify node type descriptions

Callers use a boolean to request plural node descriptions, which
obscures the intended grammatical form. Give the description API
a descriptive name and an explicit singular/plural choice while
preserving the existing generated wording.

Task-number: QTBUG-151328
Pick-to: 6.12
Change-Id: I7663c6413df56ccafd27fe82a65b224baf124e28
Reviewed-by: Topi Reinio <topi.reinio@qt.io>
Reviewed-by: David Boddie <david.boddie@qt.io>